The invention generally relates to a combined heat and power fuel cell system and associated methods of operation.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage of up to about one volt DC. For purposes of producing much larger voltages, multiple fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow field plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow field channels and orifices to, as examples, route the reactants and products through the fuel cell stack. A PEM is sandwiched between each anode and cathode flow field plate. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow field channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair of catalyst layers are often referred to as a membrane electrode assembly (MEA). An MEA sandwiched by adjacent GDL layers is often referred to as a membrane electrode unit (MEU).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. The amount of a reactant supplied may be referred to in terms of xe2x80x9cstoichxe2x80x9d. For example, for a given electrical load on a fuel cell, one stoich of hydrogen and one stoich of air would refer to the minimum amount of each reactant theoretically required to produce enough electrons to satisfy the load (assuming all of the reactants will react). However, in some cases, not all of the hydrogen or air supplied will actually react, so that it may be necessary to provide excess fuel and air stoichiometry so that the amount actually reacted will be appropriate to satisfy a given power demand.
Hydrogen that is not reacted in the fuel cell may be vented to the atmosphere with the fuel cell exhaust, and in some cases may be oxidized before it is vented. Such exhaust may also contain small amounts of hydrocarbons that xe2x80x9cslipxe2x80x9d through the fuel processor without being reacted. Substantial heat may be generated as these exhaust components are oxidized, for example by mixing them with air and passing them through a platinum-coated ceramic monolith similar to an automotive catalytic converter.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time.
There is a continuing need for systems and algorithms to achieve objectives including the foregoing in a robust and cost effective manner.
The invention provides a combined heat and power fuel cell system and associated methods of operation. Such systems are commonly referred to as cogeneration systems. In general, the system and methods of the invention relate to operation of a fuel cell system among various modes and configurations to balance heat and power demand signals. The fuel cell system is coupled to both a power sink and a heat sink. A controller is adapted to coordinate response to data signals from the power sink and the heat sink. As examples, such data signals from the heat sink may include a temperature indication or a heat demand signal (such as from a thermostat), and such data signals from the power sink may include a voltage or current measurement, an electrical power demand signal, or an electrical load.
In one aspect, the invention provides a fuel cell system that includes a fuel cell stack and a first coolant circuit. The first coolant circuit is adapted to circulate a first coolant through the fuel cell stack and transfer heat from the fuel cell stack to a heat sink. The heat sink can be any medium or object that heat is transferred to. In some embodiments, the heat sink is a hot water tank. In other embodiments, the heat sink is a body of air in a building. In still other embodiments, the heat sink is a generator portion of an adsorption cooling system. As discussed herein, other heat sink applications are also possible.
A second heat source such as a fuel processor or an exhaust gas oxidizer is present in the system and a second coolant circuit is adapted to circulate a second coolant through the second heat source to transfer heat from the second heat source to the heat sink. A controller is connected to a first pump and adapted to vary an output of the first pump, wherein the first pump is located in the first coolant circuit to drive the first coolant flow. A second pump is also connected to the controller, which is adapted to vary an output of the second pump, and wherein the second pump is located in the second coolant circuit to drive the second coolant flow. As examples, the controller can be adapted to maintain a temperature of the fuel cell stack above or below a predetermined level, or to maintain a temperature of the second heat source above or below a predetermined level.
In some embodiments, the heat sink is a heat exchanger including a first flow path adapted to receive a flow of the first coolant, a second flow path adapted to receive a flow of the second coolant; and a third flow path adapted to receive a flow of a third fluid. For example, the heat exchanger could receive cold water as the third fluid. In one portion of the heat exchanger, the water receives heat from the first fluid (e.g., fuel cell coolant) and in a second portion of the heat exchanger, the water receives additional heat from the second fluid (e.g., fuel processor or oxidizer coolant that is at a higher temperature than the first fluid). The heated water (i.e., heat sink) is then flowed to its application, in this case a hot water tank.
Preferably, at least one of the first and second coolant circuits include a radiator having a variable speed radiator fan. The radiator allows the system to expel heat to ambient when the heat is not needed by the heat sink. In some embodiments, a heat demand sensor is connected to the controller and adapted to vary a speed of the radiator fan to maintain a temperature of the heat sink above a predetermined level.
The system can further include a third heat source and a third coolant circuit, wherein the third coolant circuit is adapted to circulate a third coolant through the third heat source to transfer heat from the second heat source to the heat sink. A third pump is also connected to the controller, which is adapted to vary an output of the third pump. As an example, the second heat source can be a fuel processing reactor, and the third heat source is a system exhaust gas oxidizer, such that heat is transferred from both subsystems to the heat sink.
In another aspect, the invention provides a method of operating a fuel cell system, including the following steps: (1) transferring heat from a fuel cell to a first coolant circuit; (2) transferring heat from a second system heat source to a second coolant circuit; (3) transferring heat from each of the first and second coolant circuits to a heat sink; (4) varying a first coolant flow through the first coolant circuit to maintain a temperature of the fuel cell below a predetermined level; and (5) varying a second coolant flow through the second coolant circuit to maintain the second system heat source below a predetermined level. Such methods can further comprise selectively flowing at least one of the first coolant and second coolant through a radiator; and operating a fan to blow air across the radiator to remove heat from the radiator.
In another embodiment, the invention provides a fuel cell system having a first heat source (e.g., a fuel cell stack) and a first coolant circuit, wherein the first coolant circuit is adapted to circulate a first coolant through the fuel cell stack and remove heat from the first heat source. A second heat source (e.g., a fuel processing reactor) and a second coolant circuit are also included, wherein the second coolant circuit is adapted to circulate a second coolant through the second heat source to remove heat from the second heat source.
A first heat exchanger in the system includes a first coolant flow path and a second coolant flow path, wherein the heat exchanger is adapted to transfer heat from the first coolant to the second coolant when a first temperature of the first coolant is greater than a second temperature of the second coolant. A second heat exchanger is located along the second coolant circuit downstream from the first heat exchanger, the second heat exchanger being adapted to transfer heat from the second coolant circuit to a heat sink fluid when the second coolant in the second heat exchanger has a higher temperature than the heat sink fluid. A radiator system is provided that includes a radiator and a fan, the radiator system being located along the second coolant circuit between the first heat exchanger and the second heat exchanger. The radiator is adapted to remove heat from the second coolant circuit.
A controller is connected to a first pump that is adapted to vary a flow of the first coolant. The controller is also connected to a second pump that is adapted to vary a flow of the second coolant. As an example, the controller can be configured to vary a speed of the pump or radiator fan to maintain the heat sink fluid above a predetermined temperature.
Advantages and other features of the invention will become apparent from the following description, drawings and claims.